WASHINGTON — Nuclear talks between Iran and six world powers began on a discordant note in Kazakhstan Friday as Western officials criticized the Islamic Republic for failing to respond substantively to demands that it scale back its nuclear ambitions.

Negotiations opened in the Kazakh city of Almaty with Iranian diplomats unveiling what they described as a “comprehensive proposal” for ending international tensions over Iran’s advances in nuclear technology. The deputy leader of the Iranian delegation, Ali Baqeri, said the offer was intended to allay the concerns and ‘‘establish a new bedrock for cooperation.’’

But Western diplomats, after a break, said the Iranians put forward only a vague proposal that reflected little change. A few additional details were extracted during an afternoon session before the sides adjourned, said a Western diplomat, insisting on anonymity in describing the give-and-take at a posh Almaty hotel.

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“We had a long and substantial discussion on the issues, but we remain a long way apart on the substance,” the official said as talks broke up. “We are now evaluating the situation and will meet again tomorrow.”

US and European diplomats had expected a detailed response from Iran to a set of proposals designed to lower tensions in the increasingly volatile standoff over Iran’s nuclear program. Those proposals, which included a number of concessions favorable to the Islamic Republic, was hailed by a senior Iranian diplomat as a possible turning point during the last round of nuclear talks in late February.

But hopes for further progress appeared to fade during the opening hours of Friday’s talks. Instead of a detailed counteroffer, the Iranians appeared to be repeating talking points from a previous round of negotiations that fell apart last spring, the Western diplomat said.

“There has not yet been a clear and concrete response,” said the diplomat, who described his colleagues as somewhat puzzled by what the Iranians had put on the table. “There were some interesting but not fully explained general comments on our ideas.”

The six world powers at the talks — the United States, Britain, China, France, Germany, and Russia — have called on Iran to agree to limits on its nuclear program, including a freeze on production of a type of enriched uranium that is considered close to weapons-grade. Such a freeze would be regarded as a confidence-building measure that would set the stage for a comprehensive agreement with multiple safeguards ensuring that Iran does not use its nuclear infrastructure to build atomic weapons.

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In exchange for the freeze, Iran was offered relief from some economic sanctions, with the promise of a more substantial easing in the future. Iran, for its part, is demanding immediate relief from sanctions as part of any deal. It insists that its nuclear intentions are exclusively peaceful.